11/5/ Oxidation of Alkenes: Cleavage to Carbonyl Compounds. Oxidation of Alkenes: Cleavage to Carbonyl Compounds

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1 8.8 Oxidation of Alkenes: Cleavage to Carbonyl Compounds Ozone (O 3 ) is useful double-bond cleavage reagent Ozone is generated by passing a stream of oxygen through a highvoltage electrical discharge Ozone adds rapidly to C=C bond at low temperature to give molozonide which spontaneously rearranges to ozonide Ozonide is treated with reducing agent to convert it to carbonyl compounds Oxidation of Alkenes: Cleavage to Carbonyl Compounds If tetrasubstituted double bond is ozonized, two ketone fragments result If a carbon of the alkene is bonded to hydrogen, ozonolysis will cleave the double bond to yield an aldehyde Oxidation of Alkenes: Cleavage to Carbonyl Compounds Potassium permanganate (KMnO 4 ) in neutral or acidic solution cleaves alkenes to give carbonyl-containing products If a carbon of the alkene is bonded to hydrogen a carboxylic acid is produced If a carbon of the alkene is bonded to two hydrogens, CO 2 is formed 1

2 Oxidation of Alkenes: Cleavage to Carbonyl Compounds Alkenes are also cleaved by hydroxylation to a 1,2-diol followed by treatment with periodic acid, HIO 4. If the two OH groups of the diol are in an open chain, two carbonyl compounds result If the two OH groups of the diol are on a ring, a single, open-chain dicarbonyl compound is formed Worked Example 8.3 Predicting the Reactant in an Ozonolysis Reaction What alkene would yield a mixture of cyclopentanone and propanal on treatment with ozone followed by reduction with zinc? 8.9 Addition of Carbenes to Alkenes: Cyclopropane Synthesis A carbene, R 2 C:, is a neutral molecule containing a divalent carbon with only six electrons in its valence shell One simple method for generating dichlorocarbene is by treatment of CHCl 3 with KOH Carbenes behave as electrophiles, adding to alkenes to yield cyclopropanes 2

3 Addition of Carbenes to Alkenes: Cyclopropane Synthesis Mechanism of the formation of dichlorocarbene Addition of Carbenes to Alkenes: Cyclopropane Synthesis Dichlorocarbene carbon atom is sp 2 -hybridized with a vacant p orbital extending above and below the plane of the three atoms with an unshared pair of electrons occupying the third sp 2 lobe Addition of Carbenes to Alkenes: Cyclopropane Synthesis Reaction of dichlorocarbene with an alkene results in a dichlorocyclopropane Addition is stereospecific, meaning that only a single stereoisomer is formed as product 3

4 8.10 Radical Addition to Alkenes: Alkene Polymers Radicals add to alkene double bonds Radicals remove one electron from double bond One electron left behind yielding a new radical Polymer A large molecule built up by repetitive bonding together of many smaller molecules called monomers Cellulose (glucose polymer) Proteins (amino acid polymers) Nucleic acid (nucleotide polymer) Simplest polymerization Result when an alkene is treated with a small amount of a radical as an initiator 4

5 Initiation 1. Small amount of benzoyl peroxide catalyst is heated breaking weak O-O bonds and yielding radicals 2. Benzoyloxy radical adds to C=C bond of ethylene forming a carbon radical 3. a) One electron from C=C bond pairs up with electron of benzoyloxy radical to form C-O bond b) Other electron remains on carbon (a carbon-centered radical) Propagation Polymerization occurs when the carbon radical adds to another ethylene molecule to yield another radical Termination Chain process ends by a reaction that consumes a radical Combination of two growing chains 2-R CH 2 CH 2 R CH 2 CH 2 CH 2 CH 2 R Vinyl monomers Substituted ethylene Undergo polymerization to yield polymer with substituted groups regularly spaced in alternating carbon atom long chain Polypropylene Styrene 5

6 Polymerization of unsymmetrically substituted vinyl monomers Propylene or Styrene Radical addition steps can take place at either end of the double bond to yield: A primary radical intermediate (RCH 2. ) A secondary radical (R 2 CH. ) Similar to electrophilic addition reaction More highly substituted, secondary radical is formed Worked Example 8.4 Predicting the Structure of a Polymer Show the structure of poly(vinyl chloride), a polymer made from H 2 C=CHCl, by drawing several repeating units 8.11 Biological Additions of Radicals to Alkenes Radical vs. Electrophilic Addition Reactions Electrophilic addition Reaction occurs once Intermediate is then quenched and reaction stops. 6